1. Field of the Invention
The present invention relates to phenolic resin compositions and more particularly to a latent catalyst system for use with a phenolic resole resin which imparts improved storage stability (longer pot life) to the resole resin composition while the resin retains rapid cure characteristics at use conditions.
2. Related Art
Phenolic A-stage resins are well known to be prepared from a phenolic component and an aldehyde component which are catalytically reacted. A variety of ionizable catalytic agents are known in the art for the preparation of resole resins, including various alkali metal compounds, alkaline earth metal compounds, and amines. Residual ionizable catalyst permits subsequent heat cure of the resole resins, though at the expense of storage stability. One class of resole resins are benzylic ether resins formed by the reaction of a phenol and an aldehyde under substantially anhydrous conditions at temperatures generally below about 130.degree. C., in the presence of a catalytic concentration of a metal ion dissolved in the reaction medium. Such metal ion catalytic agents include sodium and zinc acetate, lead acetate, lithium naphthanate, lead naphthanate, lead oxide, and the like. Resole resins also can be formed using alkaline catalysts, such as the alkali metal and the alkaline earth metal hydroxides, which the art has used in forming aqueous resoles, by the aqueous reaction of a phenol and formaldehyde at a moderate temperature, subsequently neutralizing the reaction medium to a pH of about 3.8-5.3 to form insoluble, non-ionizable salts, and then stripping water, to the extent desired, from the reaction medium under vacuum at temperatures ranging up to 130.degree. C. The insoluble, non-ionizable salts may be removed by filtration or other common means prior to dehydration in order to lower the residual ionizability even more.
The preparation and characterization of some of these resins is disclosed in greater detail in U.S. Pat. No. 3,485,797. The resole resins formed in the this reference also are known as high ortho-ortho resins in that the resins are characterized by ortho-ortho linkages, compared to conventional resole resins, whether formed under anhydrous conditions or not, wherein ortho-para linkages predominate.
Phenolic resole resins can be converted to a cured network (C-stage) in the presence of hydrogen ion-type catalysts, such as typified by strong inorganic and organic acids. Though such resole resins can be cured readily in the presence of strong acid catalysts, the pot life of such acid catalyzed compositions (i.e., the time until the composition is no longer useful in a particular application), suffers from the presence of such strong acid catalysts. Thus, there is a need in the phenolic resole art for improving their pot life or storage stability, while maintaining, and even increasing the speed of cure of acid-catalyzed resole resins. In this way, the cure speed and pot life performance of phenolic resole resin compositions can match those now available from vinyl esters and polyesters, and phenolic resole resins may become a viable alternative in many applications.